They have to use enough chlorine so that it effectively disinfects the piping at the extremities of the water piping system. So then naturally your going to get a higher dose closer to the water treatment plant and a lesser dose further from the plant, as chlorine parishes rather quickly.
The design of the water system is also important with respect to chlorine, ideally you want many loops as apposed to long dead ends, so that water the water is constantly moving and not ever inching to a lengthy dead end.
Oh?
What kind of chlorinated compound is building up in the piping from the water company to my house?
The greater Victoria water supply used to be treated with chlorine, which would evaporate if left in a pitcher overnight. However, “chloramine” (a mixture of chlorine and ammonia) is now being used. Chloramine has the advantage of being persistent — i.e. it does not evaporate away quickly. That means that it remains an effective disinfectant throughout the whole water distribution system. However, it also has the disadvantage of being persistent — meaning it will not evaporate if left in a pitcher, for several days even. This doesn’t bother me, because I use a filter with activated charcoal to take it out of the drinking water. Of course, I still absorb some while showering — but, I’m not overly concerned about that.
One thing that the greater Victoria area does very well, is manage the drinking water supply. We have a protected watershed, that’s about the size of the built-up metropolitan area, and collect water in reservoirs. These reservoirs are closed to the public. No one boats on, swims in, fishes from, or camps by our water reservoirs. Water enters the treatment plant with few contaminants, and therefore requires much less chlorine, or chloramine than many other areas. In addition, the entire distribution system is flushed out twice a year. Very early on, Victoria made the decision to protect its water supply, so that it would not need to rely on disinfectants for safety.
Many cities use a lot more chlorine — you can easily taste the difference. It usually comes down to the source of the water — e.g. if they get it from a slow-moving river, they have to use more disinfectant. Similarly, if the city’s water reservoir is used as a recreation area, more chlorine needs to be added. I prefer to start with pure water, and use as little powerful disinfectants as possible. The alternatives are filtering, and bottled water (i.e. someone else filtered it for you).
Here’s a link to a Health Canada publication on chlorination of drinking water. Note the part about the risks of chlorination by-products.